San Francisco 49ers linebacker Chris Borland's decision to retire because of concerns about concussions and long-term brain damage sent shock waves through the NFL on Monday night.

The 24-year-old, who was coming off his rookie season, was considered one of the league's upcoming stars. Borland's decision highlights how concerned some players are about head injuries having a negative effect on long-term health.

"I just honestly want to do what's best for my health," Borland told ESPN's "Outside the Lines" on Monday. "From what I've researched and what I've experienced, I don't think it's worth the risk."

In addition to the lasting effects of head and brain injuries, the NFL also should be concerned about players who are retiring at age 30 or younger. In recent weeks, several young, high-profile players have decided to call it quits.

Borland's 49ers teammate, Patrick Willis, 30, announced his retirement, seemingly in the prime of his career. Jake Locker (age 26), Jason Worilds (27) and Maurice Jones-Drew (29) also have announced their retirements in recent weeks.

Here's how current and former NFL players reacted to Borland's decision:

Guys deciding to walk away from the game at a young age is a great reminder to us all that life has a bigger picture. I wish them the best!